The present invention relates to a dichromatic reading device for use with a sheet bearing information printed in various colors, which is capable of reading out information printed on the sheet in one color discriminately from that printed in other colors.
Where image information having various colors is printed on a white sheet, and it is desired to record the image from the sheet using a recording device which can record information in two colors, for example, red and black, red image information is reproduced in the red color, for example, and the remaining portions of the information colored other than red are reproduced in the black color.
FIG. 1 is an explanatory illustration of the operational principle of a conventional dichromatic reading device which has the color information function. With such a device, it is assumed that the portion of the information which is printed in red is read out as it is and the remaining portion of the information which is read out as black image information. The device includes a first image sensor which receives light reflected from the information bearing sheet and coverts it into an electrical analog signal, and a second image sensor which receives the reflected light after passing through a cyan filter, which is a complement of red and thus blocks the red color, and converts it into another electrical analog signal.
FIG. 1(a) shows the levels of pulses, each of which corresponds to a portion of the information printed in a different color, obtained from the analog signal from the first image sensor. In FIG. 1(a), the signal level corresponding to a black colored portion of the information is zero, and the signal levels corresponding to blue and green colored portions are substantially the same. The signal level corresponding to the red colored portion of the information is about twice that of the blue or green colored portion, and the signal level corresponding to a white colored portion, i.e., the background portion, is the highest. Therefore, by discriminating the output pulses from the first image sensor with a predetermined threshold level which is higher than the blue and green levels and lower than the red level, a "white" binary signal is obtained as shown in FIG. 1(b).
On the other hand, FIG. 1(c) shows the pulse levels for the portions of the information printed in various colors obtained from the second image sensor. In this case, the pulse level corresponding to the black portion is zero as in the case of the first image sensor and the levels corresponding to blue and green information are substantially the same as those output by the first sensor, since the blue and green light can pass through the cyan filter which cuts off only red light. Therefore, the pulse level corresponding to the red information becomes zero and that corresponding to the white background is lowered since the red component of the white light is also cut off. Therefore, by discriminating the output of the second image sensor with the predetermined threshold level, a "complementary" binary signal is obtained as shown in FIG. 1(d).
FIG. 2 shows an operation circuit which receives the two kinds of binary signals in FIG. 1(b) and 1(d), to obtain a red image signal and a black image signal.
In FIG. 2, the operation circuit is composed of an inverter 16 whose input terminal 12 is used as one of the inputs to the operation circuit, and a NAND gate 14 which has a first input connected to an output of the inverter 16, and a second input connected to a first input terminal 11 of the operation circuit. A first output 15 of the operation circuit is taken from the NAND gate, and the second output is taken from terminal 13 directly connected to the second input 11.
The "white" binary signal in FIG. 1(b) is supplied to the first input terminal 11 and the "complementary" binary signal in FIG. 1(d) is supplied to the second input terminal 12.
During the time when the dichromatic reading device is reading out the black, blue or green image portions of the information, the output of the first image sensor, i.e., a binary 0 is supplied to the first input terminal 11 of the operation circuit, which appears at the second output terminal 13 thereof as it is.
The second output terminal 13 is inversely related to the black image signal, and the reading device is designed to print in black when a binary 0 appears thereat.
The binary 0 is also supplied at one input to the NAND gate 14 during the time when the device is scanning either black, blue or green areas, and the NAND gate 14 thus provides binary 1 at the first output 15 of the operation circuit.
The reading device functions to print red color information upon the appearance of the signal 0 at the second output 15 of the operation circuit.
On the other hand, when the dichromatic reading device is reading out red image information, the binary 0 is supplied to the second input 12 of the operation circuit and a binary 1 signal to the first input 11. The signal 0 supplied to the second input 12 is passed to an inverter 16 by which it is inverted. The inverted signal is supplied to the other input of the NAND gate 14. In this case, a binary 1 is supplied to both of the inputs of the NAND gate 14 and therefore a binary 0 appears at the first output 15 of the operation circuit. At this time, since the signal at the second output terminal 13 of the operation circuit is 1, the reading device functions to print the red information according to the binary 0 at the first output 15.
When the reading device is reading out the white area (background), a binary 1 is supplied to the second input terminal 12 and therefore a binary 0 is supplied to the first input of the NAND gate 14 causing a binary 1 to appear at the second output 15. Since at this time the second output 13 provides a binary 1, there is no print signal obtained from the operation circuit.
In the conventional dichromatic document reading device as mentioned above, the condition indispensable to correct operation is the equality in level of the pulses corresponding to the blue or green image information from the first and second image sensor. That is, if the above condition is not satisfied, the dichromatic reading device cannot provide a correct result. For example, if the first image sensor is more sensitive than the second sensor to green, for example, and green image information is represented by a binary 1 of the "white" binary signal and by a binary 0 of the "complementary" binary signal due to some improper selection of the threshold level, the operation circuit in FIG. 2 will interpret the above situation as being the red printing condition.
Thus, in the conventional device, it is impossible to read a specific color discriminately from other colors unless the threshold levels of the two image sensors are selected suitably and individually according to the characteristics of the image sensors and the signal processing circuit. This is difficult and troublesome as a practical matter.